Until recent years, large steam generating stations have been allowed to discharge waste combustion gases to the atmosphere with little or no purification. It has become well established that the escape of such gases, and particularly the acidic oxides therein, can be injurious and harmful to the surrounding environment and its inhabitants.
Particularly objectionable are the sulfur oxides generated in relatively large amounts in the burning of high sulfur-containing coals. The gaseous pollutants may react in the atmosphere to form acids which can have a very deleterious and toxic effect on the environment. This concern about the presence of sulfur oxides in stack gases has increased in recent years due to the shortage of low sulfur fuels such as natural gas, low-sulfur coal and fuel oil.
It is known to spray an atomized lime slurry into a waste stack gas stream for the purpose of reacting the lime with the sulfur oxides in the gas stream. This may be carried out in a conventional spray dryer, producing a dry powder which can be collected in a particulate collection apparatus such as a baghouse fabric filter. For instance, it was reported in the Chemical and Engineering News of Dec. 1, 1980, page 28, that such a system has been put into operation at a boiler at the Argonne National Laboratory in Illinois, converted to burn high-sulfur coal.
A principal problem experienced with such apparatus is that the volume of stack gases produced can vary substantially over a predetermined period, dependent upon the load placed on the generator. At night for instance, the load can be substantially less than during the day, as much as a third or a quarter of the maximum generator load, correspondingly affecting the volume of stack gas produced.
At the same time, the efficiency of reaction between the alkaline or lime slurry and the sulfur oxides is dependent at least in part upon the velocity of the gas flow into which the lime slurry is introduced. One criteria therefore is to maintain as constant a gas flow velocity as possible. By closely controlling velocity and maximizing the efficiency of the reaction, one can thus optimize the ratio of lime slurry to gas flow required for the reaction. This in turn makes it possible to prevent overcooling of the stack gases and the flow into the baghouse filter, insuring that such flow will be relatively dry to avoid condensation within the filter.